1. Field of the Invention
The present invention relates to constant false alarm rate circuitry in radar systems and more particularly to a constant false alarm rate process which enhances sensitivity at ranges beyond the radar's horizon to eliminate ground clutter, sea clutter, and ground/sea vehicular traffic.
2. Description of the Related Art
State of the art doppler radars, such as airport surveillance radars, typically operate in an environment where the noise generated within its own receiver is not the dominant source of interference. Undesired echoes from rain and clutter and undesired signals from other radiating sources often exceed the receiver noise levels. These sources of interference can operate to not only obliterate the radar display, but can overload the signal processor that is tracking valid targets of interest. In order to counteract this, the signal processing involves variable detection threshold levels being applied to the digital processor which then digitally compares the power level of the cell of interest with the current detection threshold presently being used to output a signal indicating whether a target is located in the cell of interest.
At each cell, one can define the probability of detection Pd of a desired target and the probability of false alarm Pfa from noise or one of the above sources of interference. The process by which the radar automatically adjusts its sensitivity as the intensity of interference varies is called constant false alarm rate or simply CFAR.
One known CFAR process, termed range averaging CFAR, operates to set the detection threshold based on the mean noise power which is generated by summing a plurality of noise samples over range. Another well known CFAR process is called clutter mapping. This involves determining the detection threshold from a time average of a noise power in a plurality of previous scans rather than from surrounding reference cells on the same scan.
The concept of CFAR is described in detail in Chap. 3, Sec. 3.13, pp. 3.46-3.53 and Chap. 8, Sec. 8.2, pp. 8.12-8.21 of, Radar Handbook, Merrill Skolnik, 2.sup.nd Edition, 1990, McGraw-Hill, Inc. and can be referred, if necessary, for a better understanding of constant false alarm rate principles.
In U.S. Pat. No. 5,038,145, entitled, "Three Window Constant False Alarm Rate Circuit", Peter A. DeCesare, Aug. 6, 1991, and which is incorporated herein by reference, there is disclosed a range averaging CFAR for detecting targets while lowering the false alarm rate, for example, during severe weather by using three windows for calculating mean level thresholds. In addition to a lead window and a lag window, each separated from a cell of interest by a predetermined number of cells, a straddle window is used which includes the cell of interest. Each of the lead, lag and straddle windows are subjected to editing to remove three adjacent cells including the cell with the highest power level. The remaining cells are used to calculate a mean level threshold for each of the three windows. The largest of the three mean level thresholds is used, unless it is derived from the straddle window and it is significantly larger than the largest of the mean level thresholds derived from the lead and lag windows.
The present invention is an extension of this concept and results from an effort to extend the sensitivity in its constant false alarm rate circuitry for increased range performance.